You will probably need to use keyboard input (up/down keys and enter) to do this. I have found that attempting to do this with direct manipulation (by setting the values) does not fire the events correctly or at all.

If you look at the bottom of the first page you will see my post with the code for doing this. The code will select the correct optiontag no matter where it is in the list or what is selected, and it will press enter. It works for me every time, no matter what browser.